Method of grinding worms and screws



Aug. 20, 1935, N. TRBOJIEVICH 2,011,956

METHOD OF GRINDING WORMS AND SCREWS Filed July 7, 1930 s Sheets-Sheet 1w F W ATTORNEY/8' Aug. 20, 1935. N. TRBOJEVICH METHOD or GRINDING WORMSAND SCREWS Filed July 7, 1930 s Sheets-Sheet 2 INVENTOR ATTORNEYS Aug.20, 1935. N. TRBOJEVICH METHOD OF GRINDING WORMS'AND SCREWS 3Sheets-Sheet 3 Filed July '7, 1950 zum mm t I ATTORNEYS Patented Aug.20, 1935 UNITED-"STATES METHOD OF GRINDING WORMS SCREWS NikolaTrbojevich, Detroit, Mich. Application July '2, 1930, Serial No. 466,204

'1 Claims.

The invention relates to a novel method of grinding worms and screws.

Heretofore such worms were ground by means I, of a disk wheel which waspositioned in its plane a at an angle relative to theworm axis and thework was rotatedand translated relative to the wheel in a succession ofrapid passes. each pass the machine would stop and the work (or thewheel) would be returned to its initial position and another pass wouldbe taken. In order to prevent the burning or cracking of the 'work thepasses had tobe taken at a considerable rate of speed, from 15 to '70ft. per minute,

thus giving passes of a very short duration, especially so in the wormsthat were comparatively short and had a steep helix angle, as inautomobile axle worms, for instance.

I have found three fundamental disadvantages existing in the abovedescribed conventional method and have succeeded in eliminating allthree of them by improving the method of grinding as hereinafter shown.The first disadvantage, the short duration of individual operaseconddisadvantage is that the form as ground in the worm thread depends notonly upon the exact shape to which the wheel is dressed but also uponthe diameter of the wheel, the so-called helical interference. This isaconsiderable source .of trouble because the wheel is reduced in diametereach time it is dressed. thus producing slightly different workeachtime. The third disadvantage is that the wheel engages the work i with aline contact thus preventing the ground ofl particles from leaving theplace of contact instantaneously and causing overheating, clogging andscratching.

In my improved method I position the wheel in the same fashion asbefore, but I oscillate the same in its plane tangentially andtransversely first change the nature of wheel contact completely bygoing from a line to point contact; second, the produced form is nowindependent of the wheel diameter and third, the duration of each passmay now be made as long as desired and the entire stock may be removedin one pass only After I tions or passes, I have already mentioned. The

work, to save labor and wear and tear upon the 5 machine and to reducethe expense for emery wheels.

In the drawings I I Figure 1 is the transverse section of the worm takenin the plane ll of Figure 2 10" Figure 2 is the side view of the wormdiagrammatically represented;

Figure 3 is the cross section of a grinder capable of generating theworm shown in Figures 1 and 2; 5

Figure 4 is a diagram showingthegrinder and the imaginary prism matingwith the worm in plane 4-4 of Figure 2;

' Figures 5 and 6 ared'iagrams explanatory oi? the Equations 4 and 5;201' Figure 7 is a perspective view of an involute helicoid;

Figure 8 is the plan view of my improved worm grinder;

Figure 9 is the section 9-4 of Figure 8 showing 25 the feed mechanismemployed in this machine;

Figure 10 is a diagrammatic view of a globoid A v worm being ground onthis principle.

The principle upon which this invention is based will now-be explained.In Figure 1 the trans- 3o bounded on its two sides by two similar trans-35 verse tooth curves 23 which are preferably involutes developed fromthe base circle 24, but they also may be any other curves such' asArchimedean and other spirals without afiecting this broad principle. Apitch circle 25 is now selected or established and the tooth curves 23are fixed in their relative positions by assuming a thickness of tooth umeasured along the arc of the said pitch circle, while the active orcontact surface of the said curves will extend from the outside circle28 to within a short distance from the root circle 21, thus providing aclearance space at the bottoms of the worm teeth.

In order to generate a worm from the said i lamina 2| we rotate andtranslate the same at a fixed ratio about the axis 3i perpendicular toand concentric with the lamina, thus producing an infinite number ofhelixes 30 all having the same lead L, Figure 2. The lamina2l may beconsidered as a spur gear element and thus it will correctly mesh with arack 28 having a pitch line 29 tangent to the pitch circle at the pointC, a pressure angle A equal to the pressure angle of the tooth curve 23at the point B where the said curve intersects the pitch circle 25, anda tooth flank 32 conjugate to the said curve 23. The flank 32 will be a.straight line .if the curve 23 is an involute and will be a curve in allother cases.

It is now evident that inasmuch as the lamina 2| is rotated andpropagated uniformly, the rack 28 in order to remain in mesh with thesaid lamina wfll have to be moved in two mutually perpendiculardirections, i. e. along its pitch line 29 and the worm axis 3|. Thus,the rack 28 will describe a prism having an axis coincident with theline 33,-Figure 2, and an angle of inclination D corresponding to thehelical angle of the worm. If we now denote the width of space BE of thelamina with t, measured along the arc the thickness of the rack tooth BEwill also be equal to t and the pitch of the rack p=B'F will be equal towhere r is the pitch radius and n the number of teeth or lobes in thelamina.

It-stands further that 8 cos A- I (2) where a is the base radius, and

L tan D- m (3) To determine the cross section of the grinder 34, Figure3 we must know its thickness-t and its pressure angle A. From thetriangles Figures and 6, we have tan A=tan A sin D t'=t Sin D andG'C"=GC, Fig. 1

Now, the object of this invention is, first, to find the prism meshingwith the worm to be ground, second, to mechanically reproduce the saidprism by means of an oscillating grinder, third, to determine thenecessary amplitude or length of stroke of the said grinder and fourth,to rotate and translate the worm in order to finish both sides of thethread from end to end in one cut.

The next step will be to determine the lines of contact of the prism 35,Figure 4 with the worm threads. In order to do this we begin with theFigure 1 in which the two lines of action HCM and KCN are firstdetermined. These two lines will be both straight and tangent to thebase circle in the involute system, will be certain curves in any othersystem and will cross each other at the pitch point C in all systems.

When the lamina 2| is rotated in the direction of the arrow 36 the rackwill translate in unison in the direction of the arrow 31 and the pointsof tangency will move along the two pressure lines HCM and KCN, thusrequiring the projected length of stroke S to completely finish thecurves 23 from top to bottom. Therefore, if we project the said pressurelines upon the worm thread surface in Figure 2 we obtain the two skewlines NK' and HM' respectively, said lines being the locus of thesimultaneous contact of the worm thread with the prism 35 at anyoneinstant.

Although the thread surfaces 38 and 39 are warped and curved it isreadily proved that in involute system both lines of contact NK and HM'respectively are straight lines and are oppositely inclined relative tothe plane of paper, Figure 2 without intersecting each other. That theselines are straight follows from Figure 7 which diagrammaticallyrepresents another well known method of generating aninvolute helicoid.A right angle triangle PP'P rolls upon the base cylinder 40 in such amanner that its adjacent side PP rolls upon the base circle 24 and itshypothenuse P'P" rolls upon the base helix 4| thereby describing withthe apex P an involute 23 in the plane of the said base circle. Thehelicoidal surface is, therefore, composed of straight lines PP" whichare at each instant perpendicular to the tangent 32 to the involute 23,said tangent being identical with the rack tooth flank 32 as shown inFigure 1. Due to the fact that the developed length of the generator FFis exactly equal to the arc length of the helix 4| it follows from thetheory of such surfaces (developables) that the plane comprising theperpendicular lines 32 and PT" is tangent to the surface and the line oftangency is along the generator PP".

It will be of interest to note how I increase the productive ability ofthis type of the machine up to its theoretical maximum. This maximumwill be attained when the projected length of the stroke S, Figure 1will be the minimum. Therefore, I select the pitch circle 25 in such arelation to the depth of thread, that the projections of the points HNKMwill produce the minimum amplitude as measured in the direction of therack pitch line 29. This will happen when the pitch line is located at acertain point not far removed from the middle of the working depth.

To summarize the results of this investigation in the theory, we are nowenabled to select the pitch line properly i. e. such that the length ofstroke will be the minimum and the two opposite sides of the thread willbe simultaneously generated; we also may compute the length of thestroke S, Figure 4 the normal thickness 1." and the normal pressureangle A of the wheel.

Figure 8 shows the plan view of a simple and yet operative machine forgrinding worms in accordance with my invention. This machine may beconstructed from an ordinarysurface grinding machine and it will befound to be reasonably accurate and automatic (during one complete pass)in its operation.

The work table 42 of the machine reciprocates in a direction at rightangles to the wheel axis 43 in the bed 440: of the machine and thenumber, speed and length of strokes are adjusted in the same manner asin any other surface grinder. The grinding wheel 34 may be of anydesired diameter and should be rapidly rotated to have a peripheralspeed of about five to six thousand feet per minute in order to grindhardened steel. The wheel contour is V-shaped and is dressed to theexact dimensions as was shown in Figure 3. The worm to be ground 44 ismounted upon the arbor 45. It abuts the shoulder 46 with its one end andis kept tightly clamped in position by means of a nut 41 at its otherend. The rear end of the arbor 45 is formed in a mandrel 48 and carriesa master screw 49 which is tightly clamped upon the said mandrel bymeans of the nut 50. The diameter, the form of thread and the number ofthreads in-the master 49 may be and feed the worm the above mentionedfour I arbitrarily'selected; however, it is necessary that its lead beexactly the same as that of the worm 44 and its d also. must be the.same.

The master snugly fits into the nut which is preferably made of castiron lined with babbitt and is clamped by means of screw 52 in a boresituated in the upper part of the vertical shank 53 of the base plate54.

The front end of the arbor 45 is formed into a slidingspindle'55 and, isprovided with one or more longitudinal keys 56.- The feed worm gear 51is of the self-locking (single thread) type, it'

is keyed to the spindle by means or the key I8 (Figure9), and-isrotatably housed in the corresponding bore of the bracket 59, beingheldtherein position, by meansof the thrust collar II; The feed screwGHFigure 9) is formedin a "taperfiournal .62 at its upper end where itrotatabiy fits in the correspondingtaper bearing of .thebrack'et 59, anis held in position by means of the. thrust 00 r ,63. The said screw 6|snugly fits into'the teeth of the worm gear- 51 with little or ,nobacklash andis pivoted by means of the'pivot 64 in the base 65 of the sbracket 59. A ratche t 66 is keyed by means of a key 61 to the saidi'eed screw and'serves to operate the same at suitable intervals. Thesaid screw is'also operable by hand by means of the squareshank .88;formed at its upper end. The

' bracket 59 is securely bolted'to the base plate 54 by means of. bolts69.

The operation of this machine will now be ex plained'. The axis\of theworm 44 is inclined at the helix angle D relative to the axis 43 of thewheel and theworm. is reciprocated in the directions 6f the arrows I0.and II, perpendicular tothe said wheelaxis. Thus, the wheel 34 describesa prism ora rack tooth tangential to the wo thread'in the space. Uponeach stroke of the table 42. two'lines N'K' and HM respectively (seeFigure 2) will be ground in the worm thread and it we now slowly-rotateand trans-. late the worm along its axis, upon the next stroke anothertwo lines will be generated adjacent to the above mentioned lines NK'and'H'M' and also lying in the worm thread.

In the practical example shown in Figure 8, I have 100 teeth in the feedworm gear'and 16 teeth in the ratchet, thus obtaining a ratio of 1600 to1 giving a spacing of generated lines in the finished worm thread aboutfour thousandths of I an inch apart for a length of helix of 6 inches.

the screw ii, the master screw 49 will rotatm andalso advance along itsaxis in the nut 5|, thus carrying the worm to be ground 44 in therequired and precise helical path. The ratchet 66 will engage theadjustable stop I2 once during each alternate stroke in the directionof. the arrow in this design, .thus causing the ratchet 66 to ro-- tatethrough an angle corresponding toone tooth thousandths of an inch.

The ratchet feed mechanism shown in Figures 8 and 9 is only amodification or a design and it will be understood that this apparatuswill properly work also in the case when the worm 44 is rotatedcontinuously, e. g. by a train of gears from an outside source insteadof. periodically as by the ratchet. It will also be understood thatinstead of reciprocating the worm in the direction of the arrows"! and'Ii we may reciproc te the wheel 34 in the same direction withoutafiectin the principle. The contours 13 ofthe grinder will be straightlines to generate worms which are involute in their transverse section,but will be curved for all other types of worms as already mentioned. Inordinary work, the worm is first roughed out in soft, hardened, andground only in order to remove a very thin film-of metal, usually lessthan about ten thousandths of an inch thick; however, in grinding highspeed steel taps iromthe solid steel, the whole depth of thread may betaken in one cut in this method. This ability of removingzcemparativelylarge amounts of SlJOCkIlIl one pass, I consider asthe greatestadvantage of this method. and this advantage is not shared by anyother-known method of worm or screw grind- Globoid worms [may also beground by this method. As isdiagrammatically shown in Figure '10 thegloboidjworm is'of such a form that it will contact with a prism alongtwo oppositely siti. e. it is translatedin'aigloboid helix instead of ina cylindrical or ordinary helix as for common Thus both sides of thewormthread may be" ground in one pass from end to end.

When the worm, straight or globoid, is multiple threaded, each threadisground in a separate operation providing one grinding wheel only isused. In Figure 8 the worm 44 is double threaded and after grinding onethread we disengage the nut 41 and index the work in order to be able togrind the other thread without disturbing the wheel 34 in its position.

What I claim as my invention is:

1. A method of grinding worms which consists in forming a disk grinder.to a profile capable of touching an imaginary prism element along aline, said prism in turn being capable of touching the helicalsurface tobe ground along another line, in positioning thecutting plane of thegrinder to coincide with the axis of the said prism, in oscillating thesaid grinder along the said prism axis and in the plane of the grinderin a direction transverse and tangential relative to the root cylinderof they work thereby reproducing the prism and in translating the wormalong its axis in a helix untilthe thread is finished from end to end inthe form of a series of generated lines.

2. A method of grinding a helical thread consisting in forming a diskgrinder in its axial plane to a profile coinciding with theinormalsection of an imaginary prism member which member is capable of.touching the worm thread surface along two-lines, one line on eachsideof the thread, in reciprocating. the said grinder parallel to the axisof the said prism and tangentially of fthe root cylinder of the work ina stroke exceeding in length the projected theoretical length of contactand in imparting to the worm a slow helical translation along the axis,thereby finishing two worm thread surfaces from end to end in the formof a series of generated lines.

3. A'method of grinding worms, screws and the like in which the grindingwheel is positioned in a direction tangentiar of the worm thread andtransverse relative to the axis of worm, in which the worm is slowlytranslated relative to the wheelin a predetermined helical path, inwhich the wheel is oscillated in its plane and'in a direction-tangentialof the root cylinder of the work, in which the rate of oscil lation ofthe grinder is of a sufficient rapidity to provide an adequate surfacespeed relative to the work to prevent burning and in which the length ofthe stroke is selected to be in excess of the length of engagement ofthe worm thread with an imaginary prism member which the oscillatinggrinder represents, thereby making it possible to remove by grindingcomparatively large amounts of stock in one passage of the worm in itshelical path past the grinder.

4. A method of generating helical thread surfaces having a plurality ofthread convolutions disposed about an axis in which a disk cutter isformed to correspond in form to the normal section of a rack toothcapable of meshing with the said worm, in which the cutter isreciprocated in the direction of the rack tooth axis tangentially of theroot cylinder of the work and transversely of the said worm axis and inwhich the worm is slowly translated in a helical path past the cutter tofinish the thread from end to end in a series of generated lines,thereby eliminating the efiects of the helical interference and enablingthe operator to produce an identical tooth form in the worm by using acutter of a predetermined profile but entirely disregarding the diameterof the said cutter.

5. A method of grinding worms whose threads are wound along a circularhelix and have involute cross contours as measured in the planeperpendicular to the worm axis in which a disk grinder is formed to adouble conical shape, each cone angle corresponding to the normalpressure angle of the worm to be ground, and having a thickness of diskas measured over the two cones corresponding to the normal width ofspace of the said worm; in which the grinder is positioned tangentiallyof the worm thread at an angle equal to the helix angle of the worm,.

- prising imparting a helicoidal translation to the work along its axisand rapidly oscillating the grinder in its plane of rotation in adirection tangent to the root cylinder of the work relative to the work.

7. A method of grinding worms which consists in slowly feeding the workrelative to a rotary grinder in a helical path along thev axis of thework and in relatively rapidly oscillating the grinder in its plane ofrotation in a straight line path in a transverse direction to the workaxis and tangent to the root cylinder of the work, thus constantlyshifting the point of momentary engagement whereby the heat iseffectively dissipated from the work and grinder.

NIKOLA TRBOJEVICH.

